Conventionally, there has been known an exhaust device of an internal combustion engine equipped with an oxygen sensor (see Patent Literature 1 (FIG. 1), for example).
As shown in FIG. 1 of Patent Literature 1, an exhaust passage (7) extends from an internal combustion engine (1) (numeral with a parenthesis indicating a symbol described in Patent Literature 1, the same indication applied hereinafter), a catalyst (8) which purifies an exhaust gas is provided to a middle portion of the exhaust passage (7), an oxygen sensor (27a) is arranged on an upstream side of the catalyst (8), and another oxygen sensor (27b) is arranged on a downstream side of the catalyst (8).
In an internal combustion engine (engine) having a plurality of cylinders, an exhaust gas discharged from the respective cylinders flows through exhaust pipes and, thereafter, are merged together by a merging pipe connected to a downstream side of these exhaust pipes. In this case, when the exhaust pipes extend linearly from the respective cylinders, there is a possibility that an exhaust gas discharged from the respective cylinders is not sufficiently stirred in the inside of the merging pipe. Eventually, data obtained by an oxygen sensor which is arranged in the inside of the merging pipe and detects oxygen in an exhaust gas is likely to become data for a local area in the merging pipe.
On the other hand, when a catalyst and an exhaust gas chemically react with each other, the difference in chemical reaction speed arises depending on portions of the catalyst. Accordingly, when an oxygen sensor is arranged on a downstream side of the catalyst, unless an exhaust gas which reacts with a catalyst is sufficiently stirred, data obtained by the oxygen sensor is likely to become data for a local area in the merging pipe. Accordingly, in the case where an oxygen sensor is provided upstream and downstream of a catalyst, it is desirable to homogenize an exhaust gas by sufficiently stirring the exhaust gas.
In mounting an exhaust device on a motorcycle, there may by a case where an engine of the motorcycle is exposed to the outside of the motorcycle. In a vehicle where an engine is exposed to the outside of the vehicle, there may be a case where the exhaust device is exposed to a natural environment such as rain and a traveling wind. When the catalyst is exposed to rain or a traveling wind, the exhaust device is likely to be cooled and hence, there is a possibility that purification performance of the catalyst is lowered. Accordingly, when a temperature around the catalyst is unknown, it is necessary to use a catalyst having a volume slightly larger than usual by taking into account an amount of lowering of purification performance of the catalyst caused by an external environment. Eventually, the catalyst becomes enlarged.
There has been a demand for an exhaust device of a motorcycle which can detect oxygen in an exhaust gas in a state where the exhaust gas is sufficiently stirred, and also can realize the miniaturization of a catalyst.